Three-dimensional structure of involutions formed in a late Pleistocene tephra layer, northeastern Japan

• Published in: Geomorphology (Amsterdam) Vol. 52; no. 1; pp. 131 - 140
• Main Author: Koiwa, Naoto
• Format: Journal Article Conference Proceeding
• Language: English
• Published: Amsterdam Elsevier B.V 30.05.2003; Elsevier
• Subjects: Earth sciences; Earth, ocean, space; Exact sciences and technology; Involutions; Last Glacial age; Marine and continental quaternary; Northeastern Japan; Periglacial loading; Seasonally frozen ground; Surficial geology; Far East; Japan; Japan, Honshu, Iwate Prefect., Kitakami; Periglacial loading; Last Glacial age; Northeastern Japan; Seasonally frozen ground; Involutions; pumice; periglacial environment; networks; drainage basins; Cenozoic; soils; volcanic ash; thawing; density; Quaternary; volcanic rocks; periglacial features; igneous rocks; pyroclastics; upper Pleistocene; freezing; permafrost; Asia; frozen ground; Phanerozoic; seasonal variations; Pleistocene
• ISSN: 0169-555X; 1872-695X
• DOI: 10.1016/S0169-555X(02)00253-2

The involutions in the middle Kitakami River basin in northeastern Japan are formed between the Murasakino Pumice (MP) layer, which fell on the area during the Last Glacial age, and overlying brown volcanic ash soil. These involutions are the festoon and/or injection type, in which the MP moved upward and brown volcanic ash soil sunk down. This paper attempts to reconstruct the three-dimensional structure of the involutions by examination of several horizontal and vertical forms at the lower, middle, and upper parts of the involutions. In the horizontal sections at the lower part, numerous circular and elliptic forms filled with a brown volcanic ash soil in the MP are observed. At the middle part, the circular forms are generally larger than those of the lower part. In addition, the MP exhibits circular or elliptic forms in the brown volcanic ash soil. It is also observed that the MP decreases upward in the vertical sections. The reconstruction shows that the MP shows an upturned structure vertically (more than 30 cm in height), and an irregular horizontal network circumscribing circular and elliptical forms, each of which is 0.5–3 m in the long-axis diameter. This structure is interpreted as load structures that result from density inversion during thawing of seasonally frozen ground (periglacial loading).